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Using Models as a Guide for Managing
Naturally Ventilated Buildings
---------------------------------------
ISU Publication #: none - electronic only
Authors: Steven J. Hoff, Ph.D. and Jay D. Harmon, Ph.D., P.E.
Department of Agricultural and Biosystems Engineering
Iowa State University
Date: 11/94
No model can ever replace the intuitive perceptions and experiences of a building operator. Models describing the natural ventilation process can be used to help understand the major factors affecting building performance and to help guide management decisions.
A major portion of IEC project 92-07 and 92-07-01 was to develop a model to help producers, building managers, and building designers base building management decisions. The model was developed to allow users of the program to investigate the following features of naturally ventilated buildings:
* Building size and eave/ridge height
* Wall and roof insulation levels
* Curtain/door size and insulation levels
* Curtain/door opening levels
* Ridge opening
* Pig density and size
* Building orientation (ridge line direction)
* Wind speed and wind direction
* Outside temperature and relative humidity
* Supplemental heater input
* Air infiltration levels
The program will output the following parameters:
* Inside average air temperature
* Inside average relative humidity
* Building heat loss
* Animal heat loss
* Building ventilation rate
* Building air exchange rate
* Expected airflow patterns in building
In addition to the computer program, a 1:18 scale model building was developed to demonstrate management of curtain and ridge openings and the overall affect on fresh air distribution.
Both models, including some example applications, will be outlined in the following sections.
Computer Model Simulation
The computer model was implemented to review some of the principles discussed in the previous NV fact sheets. For demonstration purposes, building three (see NV Fact 5) was analyzed for a hypothetical cold-weather ventilation condition. The model was
then used to suggest ventilation changes needed to improve the environment.
EXAMPLE: Cold-Weather Ventilation
For Iowa, it is not uncommon to experience periods of high wind speeds (20 MPH) at low temperatures. A naturally ventilated building, using pure naturally ventilated techniques, will struggle to maintain inside temperatures at desired conditions. Let's assume the following building conditions as an initial starting point:
outside air temperature: 32 oF
wind speed: 20 MPH
wind direction: Due North
pig count: 450
average pig weight: 160 lbs.
ridge opening: 12 inches
south-side opening: 4 inches
north-side opening: 4 inches
wall R-value: 10
roof/ceiling R-value: 15
curtain R-value: 1 (uninsulated)
infiltration factor: 1.0
In addition, the model has a feature that characterizes air infiltration. Air infiltration is air that enters the building through unplanned openings. Any outside air that enters the building will add to the heating load on the building. If we ignore infiltration, we could overpredict the inside temperature and air humidity levels. For many buildings, air infiltration can result in an additional 1.5 air exchanges per hour in the building. For purposes of demonstration, we will assume an infiltration of 1.5 air exchanges per hour, which, for this model is designated as an "infiltration factor" of 1.0.
If we incorporate the above conditions into the model, the predicted building response becomes:
inside temperature: 35.8 oF
inside relative humidity: 58 %
building heat loss: 6,616 BTU/hr
building ventilation rate: 56,736 CFM
fresh air exchange rate: 70 ACH
The air exchange rate implies that 70 building volumes worth of fresh air will enter the building every hour. This rate is representative of that recommended during summer conditions. During the winter, an exchange rate between 4 and 6 ACH, properly distributed, will maintain moisture and gas levels at acceptable levels.
The resulting temperature (35.8 oF) using the current building parameters is much below the desired level of 60 oF. We can try various strategies to improve the environment. Certainly, due to thermal buoyancy, the large ridge vent opening is allowing too much warm air to escape and hence allowing too much cold air to enter the building. As a starting point, let's reduce the ridge vent to a 4 inch opening and the windward (north-side) curtain to a 1 inch opening. Making these changes results in the following building conditions:
inside temperature: 42.8 oF
inside relative humidity: 58%
building heat loss: 17,036 BTU/hr
building ventilation rate: 19,476 CFM
fresh air exchange rate: 24 ACH
Which is still well below the desired set-point temperature. We have significantly reduced the ventilation rate (70 vs 24 ACH) but are still well above the minimum cold weather rate required.
As a second adjustment, close the ridge to a 1 inch opening, close the leeward (south-side) curtain to a 1 inch opening, and further reduce the windward curtain to a 1/2 inch opening. The resulting building conditions become:
inside temperature: 52.5 oF
inside relative humidity: 73%
building heat loss: 32,886 BTU/hr
building ventilation rate: 8,045 CFM
fresh air exchange rate: 10 ACH
Now assume that you have discovered tears in both north and south-side curtains. After repairing the tears, you have significantly reduced your infiltration rate thus allowing less cold outside air from entering the building. Let's assume that your infiltration factor has been reduced to 0.25, implying now that 0.25 x 1.5 = 0.375 ACH of infiltration air is entering. Making this change results in a building condition of:
inside temperature: 54.2 oF
inside relative humidity: 70%
building heat loss: 35,383 BTU/hr
building ventilation rate: 7,198 CFM
fresh air exchange rate: 9 ACH
However, you are still unsatisfied with the temperature and you are concerned about the rise in relative humidity. Thus, as a final adjustment, you decide to supply 50,000 BTU/hr of supplemental heat to the building. Making this final correction results in a building condition of:
inside temperature: 58.0 oF
inside relative humidity: 64%
building heat loss: 41,358 BTU/hr
building ventilation rate: 7,198 CFM
fresh air exchange rate: 9 ACH
The final conditions are now very close to the desired environmental parameters. Temperature is within 2 oF of desired and the relative humidity is within the desired range of 50-70%. We needed to add supplemental heat to raise temperature. The increased temperature, resulting from the supplemental heater, increased the moisture holding capacity of the air in the building and thus the relative humidity dropped to acceptable levels.
The model, as demonstrated above, will be available to all area Agricultural Engineering Field Specialists by January 1995. It can, as demonstrated above, help guide you when making building management decisions. It is of course no substitute for experience and knowledge that one gains from working with a building on a daily basis.
Simulating Air Distribution using Models
In addition to the computer model demonstrated in the previous section, a physical model representing a 1:18 scaled building was developed. The purpose of the physical model is to help producers and building managers visualize the flow of fresh air in naturally ventilated buildings. The model represents the swine finishing house at the ISU Swine Nutrition and Management Research Center. The model is currently designed to test three roof styles and various curtain and ridge openings. An open-gable,
inverted-V, and square chimney designs are currently used for demonstration purposes.
The heating effect of animals is simulated using a heated floor. Many different animal distributions can be simulated since the floor is divided into four individual heating sections across the length of the building.
Visually experiencing the flow of fresh air in naturally ventilated buildings helps to understand the consequences of various inlet control strategies. The model clearly demonstrates airflow by injecting a chemical (titanium tetrachloride) into the model. The smoke that is generated follows the path of entering fresh air. A beam of light helps to highlight the airflow patterns.
What follows is a brief discussion of the models capabilities and some features of airflow in naturally ventilated buildings. The model was fitted with an inverted-V roof style identical in shape to the ISU swine finishing facility. Figure 1, as shown below, is an example airflow with both the north and south-side curtains open 1/2 way and the ridge completely open.
Figure 1
The floor was heated to simulate the heating effect of animals. Air near the floor becomes heated and begins to rise. With an open ridge, this heated air will be exhausted through the ridge as is clearly shown in Figure 1. If air is allowed to escape from the building, the fresh outside air must enter to make-up the exhausted air. Since, for this example, the curtains are open on both sides, fresh air enters along both side walls and is distributed throughout.
Figure 1 also highlights a common ailment in naturally ventilated buildings. If air entering the building is cold (relative to the animals surface temperature) it will fall prematurely. Cold entering air falling on animals creates an uncomfortable, drafty
environment. As shown in Figure 1, the entering fresh air on the left curtain is falling to the floor before it has a chance to properly temper. Since pigs will dung where they are most uncomfortable, the floor area where incoming air is dropping will be a predominant dunging zone. A correction for this problem is to ensure that cold winter air enters the building as close to the eave as possible. This will direct airflow far above the animals and increase the likelihood that cold outside air will temper before reaching animals.
Figure 2 represents a different inlet control strategy with detrimental consequences. During winter periods, the temptation is to close down as many inlets as possible to maintain temperature control. The example shown in Figure 2 represents a control strategy where one of the sidewall curtains (right side) is completely closed and the ridge vent is completely closed. This is a dangerous practice. If we want fresh air to enter the building, then the single sidewall opening (left side) that we have opened must behave as both an inlet and an outlet. In fact, the bottom half will behave as an inlet and the top half will behave as an outlet.
Therefore, fresh air entering along the bottom half of the opening will essentially circle back and be immediately exhausted to the outside along the top half of the inlet. This problem is called short-circuiting and as the name implies, fresh air is short-circuited to the outside before it has a chance to dilute moisture, dust, and gas levels in the building. Temperature control may be fine, but air quality conditions in the building will become intolerable.
As clearly shown in Figure 2, the smoke introduced with the incoming air exhausts at the top of the opening and never becomes distributed in the building. This can become a serious problem during cold-weather ventilation control of naturally ventilated buildings. During winter periods, it would always be recommended to leave the ridge open, sometimes very slightly, to allow an additional escape path for stale inside air.
Figure 2
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